1. Field of the Invention
The invention is related to a calibration phantom. Especially it refers to a drum-type source calibration phantom with different densities to derive the calibration curve for density and counting efficiency. According to the counting efficiency from the multi-density source calibration phantom corresponding to the density of the sample of nuclear waste, it is a source calibration phantom to derive accurate measurement results when it measures total Gamma activity or specific activity for different nuclear waste materials.
2. Description of the Prior Art
Until now, the equipment for the total gamma activity measurement for bulk nuclear waste with extremely low activity is Waste Curie Monitor, a large-area plastic scintillation detector assembly. Its advantages include high radiation sensitivity, high counting efficiency, short counting time, unlimited waste volume and displayable weight for waste samples.
However, the calibration method for traditional activity monitor has the following shortcomings: (1) underestimate or overestimate for activity because usually it only considers weight but not material density and causes radiation self-absorption effect; (2) counting efficiency calibration is inapplicable because usually it uses single material in monitor shielding to establish density efficiency and ignores that the sample is not single component material; (3) sample measurement position is different because usually the sample is placed under the shielding, which is different from the geometric center for efficiency calibration; (4) sample volume is different; usually volume is not limited, so different distance to scintillation detector during efficiency calibration causes errors in activity analysis; it does not meet the requirement for accuracy in waste activity analysis by radiation safety administrator; (5) there is no correction for the calculation for the radiation energy for various radionuclides in radioactive waste and the Gamma activity for multiple radionuclides; it causes errors in total activity measurement.
In recent years, researchers further develop new calibration methods to replace the original method for point source efficiency at the geometric center of the activity monitor. They measure total Gamma activity for the wastes to determine classification of the wastes and non-radioactive wastes. Current efficiency calibration methods for plastic scintillation detector include: (1) US Themo-Eberline uses transmission factor to correct the self-absorption effect for different standard mass; the formula is TF=net counting with shielding source/net counting without shielding source; generally TF≦1.0; assuming no shielding source in compensation air for calibration efficiency in geometric center; let the transmission factor parameter for the established water phantom calibration efficiency be 1; enter weight to mass parameter (every 10 kg in one unit); different sample weight will be correlated to that with transmission factor=1 and obtain total Gamma activity after correction; (2) German RADOS uses single-material metal plate assembly for multi-density calibration efficiency; (3) Japan Nuclear Energy Safety Organization's metal pipe and metal plate assembly for calibration efficiency for multiple radionuclides; (4) US NE Technology uses assembly of multi-radionuclide point source and single-material Brazil logs (density=1) for multi-weight (0˜60 Kg) calibration efficiency. However, current correction methods still only consider approximate weight and geometry without sufficient correction for self-absorption effect and various factors in mass and energy reactions, so they fail to obtain accurate total Gamma activity.
In summary, it is necessary to have a drum-type volume source calibration phantom and measurement and calibration methods to solve the issues with traditional methods.